ci: add automatic POT updates to dev branch

[greatislander]

This PR uses the shivammathur/setup-php action to install WP-CLI, the WP-CLI i18n command, and Pressbooks CLI. It then runs the wp pb make-pot command to generate an updated .pot file for the plugin.

Notes

This action will only run:

• On the base repository (not on forks)
• On the dev branch
• When either .js or .php files have been modified (these are the only sources for localizable strings so the action can be skipped if neither file type has been modified)

Currently, this action does not automatically commit the updated .pot file. Instead, it attaches it as a workflow artifact (see here for an example: https://github.com/greatislander/pressbooks-lti-provider/actions/runs/757127517). These artifacts can be compared to the existing .pot file over the coming weeks, and once the dev team is satisfied with the consistency and accuracy of the results, these instructions can be followed to enable auto-commit and remove the workflow artifact upload step.

[codecov]

Codecov Report

Merging #123 (a454176) into dev (0aa76d3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                dev     #123   +/-   ##
=========================================
  Coverage     79.07%   79.07%           
  Complexity      327      327           
=========================================
  Files             9        9           
  Lines          1171     1171           
=========================================
  Hits            926      926           
  Misses          245      245           

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[ho-man-chan]

LGTM